As implied by the mysterious voice that speaks in the prologue, the date "December 23rd" means something to the Doctor. The voice encourages them to remember what happened back then even if it sends them into peril.

December 23
As implied by the mysterious voice that speaks in the prologue, the date "December 23rd" means something to the Doctor. The voice encourages them to remember what happened back then even if it sends them into peril.